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Why are modern rotor blades swept on their outer part?

  • A

    To increase the value of the drag divergence Mach number.

  • B

    To increase the main rotor thrust at take-off.

  • C

    To lower the tangential velocity of the blade tip.

  • D

    To decrease the value of the drag divergence Mach number.

The drag increase, caused by the wave drag, is intrinsic to the formation of shockwaves and cannot be eliminated. At precisely MCRIT, there is little drag rise, but very shortly afterwards, drag begins to rise steeply. The point at which this happens is known as “Drag Divergence Mach Number.”

Wave drag is composed of two parts: energy drag (energy lost in the temperature rise through shockwaves) and boundary layer separation at the foot of the shockwave.

While the shockwaves cannot be prevented from forming, by careful designs their effects can be reduced. Swept tips fitted to helicopter blades (like BERP blades) reduce drags and delay the onset of the drag rise, thus, allowing higher rotor speeds without reaching MCRIT and as a result the Drag Divergence Mach Number, since they happen at higher free stream Mach number (MFS) delaying the onset of some of the Mach effects.

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